Analyzers for digital transmission networks such as local area networks (LANs) and wide area networks (WANs) are well known. Networks have many different formats or protocols in which they convey digital data. The digital data are usually transmitted in packets or frames which are usually of varying length, depending upon the number of bits in the data portion of the packet. When the protocol dictates that the packets be of uniform length, the packet is usually called a "cell."
Packets usually have headers (e.g., addresses) and footers on the two ends of the packet, with the conveyed data bits being in the middle. The nature and content of the headers and footers is usually dictated by the protocol of the type of network. Network analyzers are expected to monitor the digital traffic or bit stream so as to identify and examine principally the headers and footers of each packet in order to analyze the digital health of the system. Hence, they are often called network protocol analyzers. There are many examples of network protocol analyzers, an exemplary one is shown in U.S. Pat. No. 4,792,753 granted to Iwai on Dec. 20, 1988.
Any place giving access to the network is called a "port." In order to analyze what is happening to the bit stream between any two ports, an analyzer must be connected to each port. A test packet might be injected at one port and analyzed as it passes the other port. However, there are situations--as when a test packet might not be sufficiently representative of normal traffic--in which using a test packet is not desirable. In that case, it may be best to analyze a normal, data-containing packet.
In order to analyze the propagation of a random packet propagating between two ports of the network, The packet is time stamped as it passes the first port and its header stored. The time stamp is not added to the packet and continued on the network. The time stamp is stored at the analyzer along with the header. When that packet passes the second port, it is again time stamped; and the two time stamps are compared to determine how long it took that packet to propagate from the first port to the second port. That propagation might have involved passing through one or more digital switches or some other network components with propagation times that have no relationship to the speed of light.
There must be some way for the time stamp information--and perhaps the header--to be transmitted from one analyzer to either another analyzer for comparison or from two or more analyzers to another computer for comparison. This can be done perhaps on the network under test or perhaps off the network under test. Flexibility of operation is very important. Therefore, analyzers usually have considerable software control of their many analysis functions. Such software control is exercised with a main central processing unit (CPU), which is usually a microprocessor contained within the network transmission analyzer, itself. A network analyzer may also have separate computer, such as a "laptop," controller to facilitate human interface and to "program" each analysis situation into the analyzer.
Also, the two analyzers must have their clocks synchronized both as to a base count coordination and to clocking together at the same rate, or the difference between the two time stamps might reflect clock discrepancy more than packet propagation time.
One way of clock synchronization and counter coordination is to put two analyzers in the same cabinet with the controller computer and use the controller computer's clock to run both analyzers and also to synchronize the counts used by both analyzers. This is the technique used in the DA-30 network analyzer manufactured by Wandel & Goitermann Technologies, Inc, of Research Triangle Park, N.C. However, it is sometimes necessary to analyze the propagation of packets as they pass more than two ports on the network or when the ports are physically far enough apart so that connecting to two ports of the network to the same cabinet becomes inconvenient.
Another way of clock synchronization is to connect each analyzer to a satellite radio receiver to receive time signals from the Global Positioning System satellites. Such an option is available from Wandel & Goitermann Technologies, Inc., for use with their DA-30 analyzer. However, such synchronization may not be sufficiently accurate for smaller networks for which the satellite time differences may be a substantial percentage of the expected propagation time through the network.